This is one of the best knees in ages. It's of the super realistic variety. The actor is wearing a cup and the actress does not hold back in any way and knees him full force. The bust occurs in the 4th minute of the movie during the opening credits.
Which Actress?

Close-up shot of Mia (KB) grabbing Chris (Chris Klein) crotch, and then the girl is riding thim, then she rolls off him while he's still inside her, causing him pain. It's more of a dick-bust versus a ballbust. shortly afterwards stiffler headbutts a guy in the balls
Anyone cap or link? -- Clip: http://femdomspace.com/videos/play/femdomspace/12989/american-reunion-2012-ballbusting-scenes

Wearing a really nice outfit, Cynthia fights a bunch of men one-on-one. Her fourth opponent goes for a high kick but Cynthia(CR) ducks under it and lays on the ground where she proceeds to kick him in the balls three times. He falls to the ground groin cupping as everyone watching holds themselves feeling sympathy pain. Cynthia just stands up and grins. CLIP: http://www.youtube.com/watch?v=rKBNp3yfxAI 6:05

Mother vs. Son boxing match.
Son gets sent to the canvas several times from his mother's surprisingly good punches, but he shoves her into the ropes and finally gets a punch in that sends his mother to the canvas. Woozy, she beats the count, and tricks her son into dropping his gloves, then lifts a hard kick into his nuts. Son drops to the canvas, briefly tries to raise his head up but the pain is too much and he passes out, losing the boxing match with his mom by knockout. https://www.youtube.com/watch?v=-OjCPZdgz_8&index=4&list=PLgMO8Gevw_ydchZ5NyopsZigCWBo641Qk 2:33y